From e3351b3d70cde2e4a8085791029d983cef8afec4 Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Mon, 1 Jun 2026 20:07:42 -0400
Subject: [PATCH] Add settings package: Settings file load/save/seed

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 internal/settings/settings.go      | 80 ++++++++++++++++++++++++++++++
 internal/settings/settings_test.go | 60 ++++++++++++++++++++++
 2 files changed, 140 insertions(+)
 create mode 100644 internal/settings/settings.go
 create mode 100644 internal/settings/settings_test.go

diff --git a/internal/settings/settings.go b/internal/settings/settings.go
new file mode 100644
index 0000000..f49e8e6
--- /dev/null
+++ b/internal/settings/settings.go
@@ -0,0 +1,80 @@
+// Package settings persists the daemon's user-editable configuration as a single
+// JSON file (~/.antidrift/settings.json), parallel to the store snapshot. It is a
+// leaf type package: it imports nothing else in the app so any layer may depend
+// on the Settings value without pulling in adapters.
+package settings
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+)
+
+// ErrInvalidBackend marks a settings value whose ai_backend is not a known
+// backend. The applier returns it (wrapped) so the HTTP layer can map it to 400
+// without importing the ai package.
+var ErrInvalidBackend = errors.New("settings: invalid ai backend")
+
+// Settings is the user-editable configuration. Field names mirror the env vars
+// they replace: ANTIDRIFT_AI_BACKEND, ANTIDRIFT_MARVIN_CMD, ANTIDRIFT_KNOWLEDGE_FILE.
+type Settings struct {
+	AIBackend     string `json:"ai_backend"`
+	MarvinCmd     string `json:"marvin_cmd"`
+	KnowledgePath string `json:"knowledge_path"`
+}
+
+// DefaultPath returns ~/.antidrift/settings.json.
+func DefaultPath() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".antidrift", "settings.json"), nil
+}
+
+// Load reads a settings file. A missing file is an error; callers treat that as
+// "first run" and seed instead.
+func Load(path string) (Settings, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return Settings{}, err
+	}
+	var s Settings
+	if err := json.Unmarshal(data, &s); err != nil {
+		return Settings{}, err
+	}
+	return s, nil
+}
+
+// Save writes settings atomically (temp file + rename), creating the directory
+// if needed. Mirrors store.Save.
+func Save(path string, s Settings) error {
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	data, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return err
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, data, 0o644); err != nil {
+		return err
+	}
+	return os.Rename(tmp, path)
+}
+
+// SeedFromEnv builds a Settings from the legacy ANTIDRIFT_* env vars, applying
+// the same defaults the daemon used before the settings file existed. An unset
+// AI backend defaults to "claude" (matching ai.NewBackend("")).
+func SeedFromEnv() Settings {
+	backend := os.Getenv("ANTIDRIFT_AI_BACKEND")
+	if backend == "" {
+		backend = "claude"
+	}
+	return Settings{
+		AIBackend:     backend,
+		MarvinCmd:     os.Getenv("ANTIDRIFT_MARVIN_CMD"),
+		KnowledgePath: os.Getenv("ANTIDRIFT_KNOWLEDGE_FILE"),
+	}
+}
diff --git a/internal/settings/settings_test.go b/internal/settings/settings_test.go
new file mode 100644
index 0000000..b283187
--- /dev/null
+++ b/internal/settings/settings_test.go
@@ -0,0 +1,60 @@
+package settings
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSaveLoadRoundTrip(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "settings.json")
+	want := Settings{AIBackend: "codex", MarvinCmd: "uv run am", KnowledgePath: "/tmp/k.md"}
+	if err := Save(path, want); err != nil {
+		t.Fatalf("save: %v", err)
+	}
+	got, err := Load(path)
+	if err != nil {
+		t.Fatalf("load: %v", err)
+	}
+	if got != want {
+		t.Errorf("round trip = %+v, want %+v", got, want)
+	}
+}
+
+func TestLoadMissingFileIsError(t *testing.T) {
+	_, err := Load(filepath.Join(t.TempDir(), "nope.json"))
+	if err == nil {
+		t.Fatal("want error loading missing file, got nil")
+	}
+}
+
+func TestSeedFromEnvReadsVars(t *testing.T) {
+	t.Setenv("ANTIDRIFT_AI_BACKEND", "codex")
+	t.Setenv("ANTIDRIFT_MARVIN_CMD", "uv run am")
+	t.Setenv("ANTIDRIFT_KNOWLEDGE_FILE", "/tmp/k.md")
+	got := SeedFromEnv()
+	want := Settings{AIBackend: "codex", MarvinCmd: "uv run am", KnowledgePath: "/tmp/k.md"}
+	if got != want {
+		t.Errorf("SeedFromEnv = %+v, want %+v", got, want)
+	}
+}
+
+func TestSeedFromEnvDefaultsBackend(t *testing.T) {
+	t.Setenv("ANTIDRIFT_AI_BACKEND", "")
+	t.Setenv("ANTIDRIFT_MARVIN_CMD", "")
+	t.Setenv("ANTIDRIFT_KNOWLEDGE_FILE", "")
+	got := SeedFromEnv()
+	if got.AIBackend != "claude" {
+		t.Errorf("default backend = %q, want claude", got.AIBackend)
+	}
+}
+
+func TestSaveCreatesDir(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "sub", "settings.json")
+	if err := Save(path, Settings{AIBackend: "claude"}); err != nil {
+		t.Fatalf("save: %v", err)
+	}
+	if _, err := os.Stat(path); err != nil {
+		t.Fatalf("file not written: %v", err)
+	}
+}